The present invention relates to an endoprosthesis device or intraluminal device, in particular a stent, having a covering comprising porous polytetrafluoroethylene formed by removing the siloxane from an interpenetrating network of polytetrafluoroethylene and siloxane, and to a method of making the endoprosthesis device. The stent covering can be applied on the exterior surface of the stent, on the interior surface of the stent, or both, at a thickness of as low as about 15 microns.
Endoprosthesis devices including stents, stent-grafts, grafts, vena cava filters, balloon catheters, and so forth, are placed or implanted within various body vessels for the treatment of various diseases. One particular type of an endoprosthesis device is the stent. A stent is implanted within a vessel for the treatment of stenoses, strictures, or aneurysms in the blood vessels. The devices are implanted within the vascular system to reinforce diseased, partially occluded, weakened or abnormally dilated sections of the blood vessel. Stents are often employed after angioplasty to prevent, restenosis of a diseased blood vessel. While stents are most notably used in blood vessels, they have also been implanted in other bodily vessels including urinary tracts and bile ducts to reinforce and prevent neoplastic growth.
Stents are typically longitudinal tubular devices formed of biocompatible materials and come in a variety of construction types, and are often expandable in nature. Many if not all of the materials used for stents involve metal or carbon fiber materials which are highly electro-positive and are bio-active. Since stents tend to be used under conditions were they are counteracting disease processes, supporting healing processes, or guarding against stenosis of a passage, bio-activity, which may encourage undesirable or poorly regulated growth processes, or lead to clot formation, should be avoided.
Coating of the stent can keep the stent from directly contacting surrounding tissue or fluids, and thus can theoretically protect against unwanted electrochemically induced tissue reactions.
In the field of expandable stents, a further problem arises due to the fact that many stent constructions involve structures that have numerous apertures or spaces between various strands or structural elements of the stent such as those structures that are filamentous, wire-like, or of a tubular nature in which various openings have been cut or etched into the stent. With these constructions, tissue may grow through the openings of the stent. Furthermore, the stent itself may provoke a foreign body reaction and be both a stimulus for and a framework supporting, proliferative tissue growth, resulting, for example, in scar tissue or restenosis of the very region it is placed to control.
One approach to this drawback is to provide a coating, liner, cover or both, for the stent which prevents the healing or diseased layer of tissue from directly contacting the stent, or from passing through the stent in any way. Such liners may be formed, for example, of porous polytetrafluoroethylene (PTFE) which allows the passage of fluids and vital materials while serving as a barrier to tissue growth. However, when applying such a construction, a further difficulty which may arise is that the layer or sleeve of polymer must be attached to the stent for example, by staples or sutures at one end, or is prone to developing loose pockets or folds which might accumulate organic matter or lead to sepsis or unusual growth. Also, the necessarily thin liner material may detach or degrade. The risk of loose or unattached liner material is particularly great for constructions which utilize poorly adherent polymers, such as PTFE, or structures which seek to combine an expandable stent of stiff material, which changes both its dimension and its shape, with a dissimilar liner or shell.
One method for overcoming these problems is found in U.S. Pat. No. 6,010,529 in which tube of polymeric material, e.g. expanded polytetrafluoroethylene (PTFE), is passed through the interior of a stent body and is turned back upon itself over the stent to form a cuff. The assembly is then heated and the outer layer contacts and coalesces with the inner layer, closely surrounding the stent body within a folded envelope having a continuous and seamless end. Porosity is imparted to the PTFE by previous stretching or expansion the material.
Another type of covered stent which permits radial expansion is shown in WO 96/00103. As shown and described therein, a metallic expandable stent includes an outer covering of ePTFE. The ePTFE cover exhibits suitable expansion capabilities so as to enable the cover to expand upon expansion of the underlying stent. A polytetrafluoroethylene/lubricant blend may be extruded into a tube and the tube heated to remove the lubricant. Then, in order to impart the expandable characteristics to the ePTFE cover during formation of the ePTFE cover material, the ePTFE must undergo successive processing steps of expanding the material, sintering the material, radially dilating the material and resintering the dilated material, a procedure that is quite process intensive. The device described therefore requires precise manufacturing techniques and is extremely processing sensitive. Careful processing of the material forming the cover is required in order for the cover to exhibit sufficient expansion capabilities.
U.S. Pat. No. 5,824,046 describes a composite intraluminal device, in particular an elongate radially expandable tubular stent having an interior luminal surface and an opposed exterior surface extending along a longitudinal stent axis. A stent cover is formed of unsintered ePTFE which is expandable.
There remains a need in the art to provide a stent with a cover material that is sufficiently expandable, has the requisite barrier properties and yet allows the passage of fluids and vital materials, without requiring extensive processing procedures and is thus easily manufactured and applied to the stent.
The present invention relates to a method of forming porous polytetrafluoroethylene (PTFE) without having to stretch or expand the material, and to a radially expandable endoprosthesis device covered with the solid but expandable polymer covering comprising the porous PTFE material obtained using the method of the present invention. The porous PTFE covering physically isolates the endoprosthesis from surrounding blood and tissue.
Specifically, the porous PTFE is prepared by extracting siloxane from an interpenetrating network (IPN) of PTFE and siloxane, leaving behind a porous PTFE structure without having to expand and stretch the PTFE. Consequently, the PTFE material used in the endoprosthesis device coverings of the present invention is not expanded PTFE, but it is porous.
In one embodiment the end of the prosthesis device of the present invention includes an elongate radially expandable tubular stent having an interior surface and in exterior surface extending along a longitudinal stent access. The expandable tubular stent has a stent cover on said interior surface, exterior surface or both, the cover being formed of a porous polytetrafluoroethylene. The porous polytetrafluoroethylene cover is a non-stretched porous structure, the non-stretched structure lacking note and viable structure.
In particular, the present invention relates to a radially expandable stent for use in treating stenoses wherein the stent is covered with an expandable polymer covering comprising the porous PTFE prepared according to the present invention that physically isolates the stent from surrounding blood and tissue.